1. Field of the Invention
The present invention relates to a corner reflector antenna with a ground plate having a wideband characteristic.
2. Description of the Related Art
A conventional corner reflector antenna with a ground plate has a ground plate 21, a radiator 22, a corner reflector 23, an insulator 24, and a feeding line 25 as shown in FIG. 11. The ground plate 21 is made of, for example, a rectangular metal conductor having four sides each with a length equal to about 0.6 wavelength with respect to an operating frequency. The radiator 22, insulated by the insulator 24, is perpendicularly installed on the ground plate 21 in the vicinity of its center. The radiator 22 is made of, for example, a bar-like metal conductor with a thickness equal to about 0.015 wavelength and a length equal to about 0.25 wavelength. The radiator 22 is supplied with signals through the feeding line 25. The corner reflector 23 is formed of two rectangular metal plates 23a and 23b combined together so as to form a prescribed angle. The corner reflector 23 is perpendicularly installed on the ground plate 21 so that the distance between the apical angle (the combined part between the metal plates 23a and 23b) of the corner reflector 23 and the radiator 22 is equal to about 0.35 wavelength. In this case, the metal plates 23a and 23b have a width equal to about 0.35 wavelength and a height equal to about 0.45 wavelength and are combined together so as to form an apical angle of 120°.
The corner reflector antenna with the ground plate in FIG. 11 operates as a unidirectional antenna. As shown, in FIGS. 12A and 12B, by vertical directivity and horizontal directivity with respect to the vertical polarization of the corner reflector antenna with the ground plate in FIG. 11, the direction of maximum directivity of vertical directivity coincides with a launch angle of about 27.5° with respect to a horizontal direction. The figures also show that the sensitivity decreases by about 1.5 dB.
Jpn. Pat. Appln. KOKAI Publication No. 2005-244926 discloses a UHF wideband antenna having a generally rectangular dipole element and a corner reflector provided behind the dipole element. The UHF wideband antenna uses a plate-like dipole element or a dipole element having a cavity formed in its center to balance the amplitude of an electric wave radiated from a feeding side with the amplitude of an electric wave radiated from a non-feeding side. This prevents the direction of maximum sensitivity of vertical directivity with respect to the vertical polarization from coinciding with the launch direction.
Further, with the corner reflector antenna with the ground plate in FIG. 11, an attempt to reduce the size of the ground plate 21 further increases the launch angle in the direction of maximum sensitivity, while further reducing the sensitivity in the horizontal direction. This prevents a reduction in the size of the ground plate 21. Moreover, the corner reflector antenna with the ground plate in FIG. 11 has its impedance varying significantly depending on frequency. Thus, it is difficult for the corner reflector antenna with the ground plate in FIG. 11 to offer a wideband characteristic as is apparent from a voltage standing wave ratio (VSWR) characteristic observed at a characteristic impedance of 50Ω shown in FIG. 13. FIG. 13 shows a frequency f/fo on the axis of abscissa and VSWR on the axis of ordinate. fo denotes a central frequency of the operating frequency.